Liquid roller coating lines are known in the art, and may apply solvent or water-based paints/coatings to metal strip through the use of roller-coating machines. Unfortunately, environmental regulations have made such coating lines undesirably expensive in view of the need for solvent containment and incineration systems. Additionally, there is a finite limit to the thickness of a coating that can be effectively applied using such systems.
Accordingly, powder coating of strip material has been developed in the industry. This normally involves applying electrostatically charged dry plastic powder to a strip, and then passing the strip with powder thereon through a convection oven where the powder is melted and cured through a cross-linking process. An example of a powder-coating system is disclosed in U.S. Pat. No. 5,439,704, the disclosure of which is hereby incorporated herein by reference. Reference is also made to FIG. 1 herein, taken from the '704 patent.
As shown in FIG. 1 of the '704 patent, the powder-coating system includes input region 1, powder-coating booth 3, heating chamber 5, quench 7, and output region 9. When metal strip 11 is being processed, it is suspended through booth 3 and oven 5 between a pair of entrance rolls 13 and catenary roll 15. After the powder-coated strip 11 exits booth 3, the strip enters oven 5. The thermoset powder material on strip 11 melts and cures into a coating. The curing phase involves cross-linking of molecular chains of the thermoset plastic to form the final hardened material. In one example discussed in the '704 patent, a polyester hybrid powder coated strip is held within oven 5 for approximately 25-30 seconds at a temperature of 475° F.
Unfortunately, conventional heating processes have been found to be undesirable for a number of reasons. Additionally, when gasses within the thermoset material are not permitted to exit prior to curing, the finished product may suffer from the “orange peel effect”, thus having a mottled surface (i.e. bumpy surface). This may occur when the powder-coated metal strip is heated at too fast a rate to too high a temperature. It has also been found that convection ovens are not particularly well suited for precisely controlling thermoset-coated material temperatures. Convection ovens also suffer from excessive dirt problems.
In view of the above, it is apparent that there exists a need in the art for an improved method for coating continuously moving strip (e.g. coil steel, coil aluminum, fabric, blanks, etc.) with thermoset material. There also exists a need in the art for an improved method of heating and/or curing thermoset material, so as to result in a superior finished product. It is a purpose of this invention to fulfill any and/or all of the above-described needs in the art, as well as other needs which will become apparent to the skilled artisan from the following detailed description of this invention.